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(2020) Loading and Cooling in an Optical Trap Via Hyperfine Dark States

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(2020) Loading and Cooling in an Optical Trap Via Hyperfine Dark States PHYSICAL REVIEW RESEARCH 2, 013212 (2020) Loading and cooling in an optical trap via hyperfine dark states D. S. Naik,1,* H. Eneriz-Imaz,1 M. Carey ,1,2 T. Freegarde,2 F. Minardi ,3,4 B. Battelier,1 P. Bouyer,1 and A. Bertoldi 1,† 1LP2N, Laboratoire Photonique, Numérique et Nanosciences, Université Bordeaux-IOGS-CNRS:UMR 5298, F-33400 Talence, France 2School of Physics & Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom 3Istituto Nazionale di Ottica, INO-CNR, Sesto Fiorentino 50019, Italy 4Dipartimento di Fisica e Astronomia, Università di Bologna, Bologna 40127, Italy (Received 25 November 2019; accepted 28 January 2020; published 26 February 2020) We present an optical cooling scheme that relies on hyperfine dark states to enhance loading and cooling atoms inside deep optical dipole traps. We demonstrate a sevenfold increase in the number of atoms loaded in the conservative potential with strongly shifted excited states. In addition, we use the energy selective dark state to efficiently cool the atoms trapped inside the conservative potential rapidly and without losses. Our findings open the door to optically assisted cooling of trapped atoms and molecules which lack the closed cycling transitions normally needed to achieve low temperatures and the high initial densities required for evaporative cooling. DOI: 10.1103/PhysRevResearch.2.013212 I. INTRODUCTION trap (FORT). However, the DSs are then strongly modified by the trap potential, which typically shortens their lifetime and Ultracold quantum gases have attracted much attention in eventually couples them to the light field. recent decades as versatile platforms for investigating strongly In this paper, we show that DS cooling can be used in correlated quantum systems [1] and as the basis for quantum combination with FORT when strong differential light shifts technologies exploiting atomic interferometry [2,3]. Cooling are present. We use the effect of FORT trapping light close of an atomic gas to the required temperatures requires a to the 5P / → 4D / / transitions of rubidium at 1529 nm multistage process: laser cooling in a magneto-optical trap 3 2 3 2;5 2 [13,14] to maximize the cooling action on the surrounding of (MOT); sub-Doppler cooling; loading into a conservative the trap and at its borders. We observe an order of magnitude magnetic or optical trap; evaporative cooling. Although quite improvement in the number of trapped atoms. Additionally, efficient, this process is only possible for a small subset of we explore the possibility of cooling the atomic ensemble alkali and alkali-earth-like atoms that can be initially cooled in the FORT. We achieve a notable temperature reduction of to low temperature by optical means. the trapped sample in a few ms and without losing atoms The sub-Doppler cooling phase typically uses light red and analyze the limitations of the protocol and the possible detuned from the F → F = F + 1 cycling transition of a workarounds. D line nS / → nP / [4], and can be understood in terms 2 1 2 3 2 Gray molasses commonly rely on Zeeman dark states of the “Sisyphus effect” [5–8]. Sub-Doppler cooling schemes (ZDSs), i.e., linear combinations of Zeeman sublevels not involving dark states (DSs) [9] have emerged as a powerful al- coupled to lasers blue-detuned with respect to the cooling ternative; they are known as gray molasses. Very recently they transition [15]. Generally, ZDSs are not eigenstates of the have been pivotal in obtaining an all-optical Bose-Einstein kinetic-energy operator, and free evolution turns them into condensate in microgravity [10] and a degenerate Fermi gas bright, absorbing states. Moreover, the presence of a repumper of polar molecules [11]. The DSs are coherent superpositions laser spoils these ZDSs, limiting the cooling efficacy [16,17]. of internal and external states, decoupled from the optical field However, with a configuration where the repumper and via electromagnetically induced transparency [12]; their cre- cooler frequencies are equally detuned from the excited state, ation does not require cycling F → F = F + 1 transitions, i.e., a Raman configuration [see Fig. 1(b)], hyperfine dark but can rely on any transitions of the F → F F form. states (HDSs) can be created. These HDSs have been rigor- To prevent the expansion of the cold atom cloud during ously studied in the context of coherent population trapping cooling, it is tempting to combine DS sub-Doppler cooling and electromagnetically induced transparency [9,18,19], but with spatial confinement in a far-off-resonance optical dipole not yet for gray molasses cooling. The existence of these HDSs depends on the number of degenerate ground (Ng) and excited (Ne) Zeeman states [9,15]: > *[email protected] (i) for Ng Ne, a DS always exists provided the laser frequen- †[email protected] cies match the Raman condition; (ii) for Ng Ne, additional conditions on the complex Rabi frequencies must be satisfied. Published by the American Physical Society under the terms of the For instance, DSs where the connectivity between ground and Creative Commons Attribution 4.0 International license. Further excited states forms a loop exist if the summed phase of each distribution of this work must maintain attribution to the author(s) complex Rabi frequency is 0 mod(2π )[9,20]. These states are and the published article’s title, journal citation, and DOI. particularly relevant for gray molasses cooling because they 2643-1564/2020/2(1)/013212(6) 013212-1 Published by the American Physical Society D. S. NAIK et al. PHYSICAL REVIEW RESEARCH 2, 013212 (2020) 87 FIG. 1. (a) Relevant energy levels of Rb. (b) mF -dependent light shifts on the 5P3/2 levels caused by the FORT at 1560 nm— not to scale. The blue waves show the lasers involved in the Raman scheme; 3 and 2 indicate the cooler detunings at any given point in the trap. can be eigenstates of the momentum, thus stable both under free evolution and in the presence of a slowly varying external potential. The DSs for case (ii) can exist in the usual σ+ − σ− cooling configuration, provided that the lasers tuned to the cooling F = 2 → F = 2 and repumping F = 1 → F = 2 transitions are both phased locked and retroreflected to fulfill the phase requirements on the Rabi frequencies. In addition, FIG. 2. (a) Atoms loaded into the FORT as a function of the the excited Zeeman states |F = 2, m must be degenerate cooling laser detuning from the F = 2 → F = 3 resonance during [see Fig. 1(b)], e.g., at zero magnetic field or when the con- the sub-Doppler cooling phase, for an independent repumper (empty nected states experience the same light shifts in the presence blue circles), and for a repumper in Raman condition with the cooler of far off resonance light. We used a numerical approach to (filled red circles). (b) Population fraction of atoms in F = 2 with the ascertain the exact DSs for our configuration where light at repumper in the two configurations as above; N1,2 indicate the atomic 1560 nm is used for trapping (see the Appendix). populations of the F = 1, 2 manifolds. The violet points result from a numerical evaluation of the DSs forming at each detuning (see Appendix A). II. EXPERIMENT Our experimental scheme is described in Ref. [21]. We [blue points in Fig. 2(a)] with a repumper at fixed frequency, load and cool 87Rb atoms at the center of an optical cavity on-resonance to the F = 1 → F = 2 transition; second [red used for the 1560-nm FORT. During the entire MOT stage, points in Fig. 2(a)] with the repumper phased-locked to the the FORT depth is maintained at ∼27 μK.After2sofMOT cooling laser by using an electro-optic modulator at the hy- loading, we realize a compressed MOT phase (CMOT) by perfine frequency ωhf = 6.834 68 GHz. This latter configu- detuning for 40 ms the MOT beams to =−6 to the ration leads to the Raman configuration of Fig. 1(b).We red of the F = 2 → F = 3 atomic resonance and decreasing can identify three regions: (i) ||/ < 30, where standard the optical power by a factor 10 ( = 2π6.066 MHz is the F = 2 → F = 3 sub-Doppler cooling takes place, leading natural linewidth of the Rb D2 line). Throughout this process, to optimum loading at ||/ 15 for both configurations; the MOT region overlapping the FORT remains effectively (ii) 30 < ||/ < 60; (iii) 60 < ||/, where gray molasses dark due to the large excited state light shifts. The result is occurs, respectively, on the F = 2 → F = 2 and F = 2 → a larger density in this region, in a similar fashion to what is F = 1 transitions. reported for the dark-SPOT MOT [22]. At this point we begin In the first configuration, only ZDSs can form. Gray mo- the sub-Doppler cooling phase. The FORT is ramped up to lasses leads to a 2.5 times more efficient loading at −40, 170 μK, simultaneously the cooling beams are detuned to the i.e., blue detuned from the F = 2 → F = 2 transition. This red in 200 μs, and we illuminate the atoms for 1 ms. We then coincides with an optimum in density when the cooling is turn off the cooling and repumping beams and hold the atoms applied without the FORT [7]. We find that almost all the for 500 ms in the FORT. We finally measure the number of atoms are in the F = 1 manifold [blue points in Fig. 2(b)] trapped atoms by absorption imaging. even though the ZDSs are in the F = 2 manifold. This can be understood as the effect of the repumper weakly coupling the III.
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